In U.S. Pat. No. 7,259,367 B2, rain sensing by means of a camera is proposed, said rain sensing providing extensive lighting of the passing-through window of the camera aperture angle with the pane by means of an infrared diode. The camera focus is set to almost infinite and can thus be simultaneously used for driver assistance applications. Due to the imaging on the remote range raindrops are only noticeable as disturbances in the image, which are detected by complex differential measurements of the images recorded with infrared light pulsed or modulated in synchronization with the pixel clock.
A device and a method for detecting rain are described in WO 2012/092911 A1. A camera is disposed behind a pane, in particular in the interior of a vehicle behind a windshield, and focused onto a remote region that lies in front of the pane. A lighting source for generating at least one light beam that is directed at the pane directs the at least one light beam towards the pane such that at least one beam that is reflected from the outer face of the pane impinges on the camera as an external light reflex or external reflex. The light quantity of the at least one beam or light reflex that impinges on the camera can be measured by the camera. One or more light-emitting diodes optionally with a light guide or a light band are indicated as the lighting source. If the aperture angle of the illumination is large enough, the lighting source can also be located inside the camera, e.g. on a circuit board of the camera system.
The sensitivity of the rain detection hereby substantially depends on the configuration of the illumination.